The present invention relates to a penetrometer comprising a probe with a conical nose, a cylindrical friction casing, and means for measuring axial friction forces acting on the friction casing when the probe is advanced in an area of ground which is to be analysed.
A penetrometer of this kind is known, for example, from EP 0 010 988. In this document, the penetrometer is provided with a first force transducer for measuring the forces acting on the conical nose, which is referred to below as the cone, and with a second force transducer for measuring the friction forces acting axially on the cylindrical friction casing. When the known penetrometer is pressed vertically into the ground at a substantially constant speed, the cone resistance and the friction force are measured simultaneously. As a result, a soil profile is obtained, which is desirable, for example, for designing the foundations of a building.
In order to determine the soil profile, it is also desirable to gain an insight into the horizontal strength of the ground. A generally known device for determining the horizontal strength of the ground was developed by Prof. S. Marchetti and is known as the "Marchetti Dilatometer". The drawback of this known device is that the horizontal strength of the ground can only be measured while the instrument is standing still, with the result that, in order to determine a soil profile, the device has to be pressed into the ground in steps, for example of 0.2 meter, in order in this way to obtain a sufficiently high number of measured values. For this reason, this measurement method is time-consuming. Furthermore, to determine the soil profile it is sometimes also necessary to carry out penetrometer tests.
The Cambridge Institute has developed a probe, under the name of "Cone Pressuremeter" which comprises a conventional penetrometer with a conical nose and a friction casing, on the one hand, and a pressure-measuring apparatus which is arranged one meter from the conical nose, on the other hand. The pressure-measuring apparatus comprises a diaphragm, the diameter of which can be expanded by the application of internal pressure. This "Cone Pressuremeter" is pressed into the ground in steps, in order to expand the diaphragm while the device is at a standstill, then to carry out the measurement, and finally to allow the diaphragm to deflate again, after which the probe is pressed further into the ground.
The Russian publication SU-A-548,684, published on 22 Mar. 1977, discloses a penetrometer which is designed to simultaneously determine both the cone resistance, the friction forces and the horizontal ground pressure. This known penetrometer has a cone with an associated first force transducer for measuring the cone resistance. Furthermore, this known penetrometer has a substantially cylindrical metal casing which is cut open in the axial direction. As a result, the diameter of the casing can vary under the effect of radial forces exerted by the ground. The known penetrometer is provided with a second force transducer for measuring the forces acting on the casing in the axial direction and with means for measuring the change in diameter of the casing, which change provides an insight into the horizontal pressure exerted on the casing by the ground. This known penetrometer has the advantage over the Marchetti Dilatometer that the three measurements are performed simultaneously and there is no need to move the penetrometer in steps.
The split-casing design of the penetrometer which is known from SU-A-548,684 has the drawback that it is not possible to interpret the measurement results unambiguously. As the horizontal pressure on the casing increases, the diameter of the casing will decrease. This results in an active stress condition in the ground which is characterized by a fall in the horizontal pressures as a result of the arch action in the surrounding ground. This fall depends on the frictional properties of the material of which the ground is composed. An increase in the diameter of the casing of the known penetrometer causes a passive stress condition in the ground, with the result that the horizontal pressures in the ground actually increase. As the penetrometer is pressed into the ground, the variations in the composition of the ground which arrive as the depth increases will cause the stress condition to vary constantly between passive and active, with the result that the measurement results obtained using the penetrometer cannot be interpreted unambiguously. A further drawback of this known penetrometer is that its operation can be interfered with by the penetration of groundwater and soil material.